Complete Guide to Lubricant Deposit Characterization

The current practice for the users of industrial lubricants has been to assume that all deposits or varnishes are created equal. One tends to look at the colour of a deposit and assume that, since it is brown, it must be the same as the brown sample previously observed. This assumption can be far from correct and can lead to taking the wrong corrective actions. There are many chemistry types in varnish materials.

When one thinks of sludge or varnish, the concept needs to be broadened. This material is not simply the oxidation or degradation products from the lubricant - more generally, it is the material coming out of the fluid with the potential to cause operational issues. Equipment reliability issues are not only caused by fluid degradation and varnish deposits but by any material that is not a homogenous single phase with the lubricant. Therefore, any foreign material causing deposits in the lubricant can potentially be defined in this category.

Deposit Characterization Process

The characterization of deposits becomes the path toward the root cause of its formation. There are a wide variety of testing technologies available for determining the chemistry of the deposits. However, it is sometimes challenging for operational plants to obtain samples of deposits to allow for these analyses. It is much easier to get a sample of the in-service lubricant. In this case, the first step is separating the oil degradation products believed to be responsible for generating the deposits from in-service oil samples. The separation step developed can be accomplished physically, mechanically or chemically.

The next step is to determine the organic and inorganic composition of the deposit. Two useful tests for analyzing the lubricant are elemental spectroscopy and Fourier Transform Infrared spectroscopy (FTIR). Common deposit characterization tests used in these studies are FTIR and X-ray Florescence spectroscopy (XRF). These results are then compared to provide a chemical characterization of the deposit, as well as allow for a possible root cause, to be determined. These steps are presented in Fig. 1.

Lubricant Degradation & Deposit Formation

There are many sources of lubricant degradation that often lead to deposits. Some of these include:

• Oxidation
• Thermal Degradation
• Micro-Dieseling
• Spark Discharge
• Extreme Temperature Zones
• Combustion
• Ultraviolet Degradation
• Contamination
• Incompatible Lube Or Other Liquid
• Dirt And Hard Particles
• Water
• Gas
• Additive Reaction By-Products

Once a fluid has undergone degradation or been exposed to contamination or initiation failure, there are several factors to determine the lubricant’s propensity to develop deposits. The formulation can play a large role in this. Engine crankcase formulations contain dispersants to suspend or solubilize soot and other degradation products in the fluid. The base stock of the lubricant also contributes to its solvency and the fluid’s deposit control abilities. Mineral oils have lower solvency, and synthetic API Group V fluids have higher solvency.

In addition to lubricant formulation, two other variables that determine deposit formation are temperature and pressure. These factors are particularly relevant when the degradation products are soluble and easily transition in and out of a solution. Lower temperatures will decrease a fluid’s solvency, causing some types of degradation by-products to precipitate, forming deposits. Pressure can also drive these contaminants out of solution, explaining one of the reasons why it is common to see deposits in journal and thrust bearings.

Deposit Classification Bins

There are many mechanisms that degrade lubricants and an even higher number of different chemistries found in lubricant deposits. The virtually limitless number of deposit chemistries can, however, be classified by their physical characteristics and the source of their formation. Sorting deposits into classification bins is beneficial to better understand the source of the deposits and determine appropriate remediation efforts. This article suggests some nomenclature and definitions for these Deposit Classification Bins, as shown in Fig 2.

These bins can first be defined as Level 1 in broad terms based on their chemistry, as illustrated in Fig. 3.

Better understanding the composition of the deposit allows further classifications based on the deposit formation source. The most basic chemistry difference is whether the deposit is organic or non-organic.

Non-organic deposits can be defined as those deposits that are insoluble in highly polar organic solvents and contain no carbon-hydrogen spectral features.

Organic deposits can be defined as material that contains carbon-hydrogen bonds (CH2 and CH3), is primarily insoluble in hydrocarbon solvents (which makes it a deposit) and is often soluble in polar organic solvents.

Water content is also found in many deposits. Although water is potential sediment itself, more often it is part of the deposit. It often determines the consistency and tenacity of a deposit. It is common to find moisture in deposits when they are first generated, allowing the deposits to be easily wiped off. These types of deposits are often referred to as sludge; however, they don’t necessarily contain the chemistry to make them actual sludge. Over time, these deposits may dehydrate and cure onto metal surfaces, becoming difficult to remove mechanically. These deposits are often referred to as varnish.

In contrast, sludge deposits have been shown to be those containing metal salts of carboxylic acids.

When these deposits are dried, they become powders. They physically differ from varnish in that they are always soft deposits. These deposits are classified as oxidatively derived-organic deposits with inorganic parts.

Deposits that fall into the Lubricant Bin are those components that are part of the oil formulation. Occasionally, one may find an additive component that has come out of a solution from the lubricant and deposited itself in the system. Improper blending or additive incompatibility is a prime cause of this type of deposit. Incompatibilities between two lubricant formulations can also cause one or more of the additive formulations to come out of the solution to form a deposit.

Further categorization of the deposits into the Level 2 Classification set of Bins is also possible, allowing one to group by the sources of deposit formation.

Organic Deposits

Organic deposits are often soluble in the in-service lubricant, allowing them to transition in and out of the solution depending upon the environment. These deposits are often referred to as “soft contaminants.” Organic deposits can be further divided into formulation derived, thermoplastic, thermal decomposition, oxidatively derived and contaminant derived sub-categories. This Level 2 Classification is illustrated in Fig. 4.

Formulation Derived

Many additive components may contribute to deposits either after they have reacted or due to dropping out of the solution in an unreacted state. It is common to see reaction products from sacrificial additive components in deposits. In Rust and Oxidation lubricants, it is common to find reacted primary antioxidant species in deposits, which produce organic deposits.

Thermoplastic

Some high-temperature degradation processes produce high molecular weight molecules that create deposits that act like thermoplastics. These deposits are typically solid at room temperature, however, they become liquid and flow at elevated temperatures (typically at or below fluid operating temperatures). It is common to see this type of deposit from fluids that have failed due to spark discharge.

Thermal Decomposition

Hydrocarbon molecules will typically crack at temperatures above 300°C. There are two actions that can happen. The first is when small, cleaved-off molecules volatilize from the fluid. This portion of the reaction is not a deposit former. The second is molecular condensation. As the small molecules are split off, the remaining portion of the molecule will condense. This condensation is in the absence of air, so dehydrogenation will be part of its decomposition. As a final product, the formation of coke will be observed, however, there are numerous deposit chemistries observed before the coke is formed.

Oxidatively Derived

Oxidatively derived deposits are among the most common classification found in lubricant deposits because oxidation is one of the primary lubricant failure modes. These deposits usually have a higher molecular weight than the lubricant, which contributes to their inability to stay in solution. Many incorrectly assume that most deposits fit into this category.

Contaminant Derived

Organic contaminants can ingress into the lubricant system, initiating a reaction. This reaction can be with the current formulation or it can be incompatible with the fluid, resulting in an organic contaminant deposit. This has been observed with some types of gas contaminants that may react with the in-service lubricant to create unique, organic deposits. Ammonia gas ingression, for example, has been found to react with Oxidation Derived degradation products to create deposits consisting of primary amides. Other gases have been known to produce their own signature deposits.

Biologically Derived

Deposits that are derived from biological growth include plant materials such as sugar, cotton and proteins are considered biologically derived. These are often from fermentation processes. Microbial growth can also cause deposits classified in this bin. Although these deposits are organic in nature, they are typically found with the non-organic deposits during isolation.

Non-Organic Deposits

Non-Organic deposits are also defined as a Level 2 Deposit Classification Bin. The Non-Organic Bin includes wear metals and dirt. In the non-organic classification, one finds the categories of coal and plant life. These two do contain some carbon-hydrogen functionality; however, they behave more similarly to the non-organic than the organic. In addition, coal is often misidentified as coke or soot and thus should be placed near these materials in the classification tree. When Coke is formed as a deposit, there is often a corresponding Thermal Decomposition deposit observed in the Organic Deposit Classification Bin. The synergism between these two allows improved identification of the source and thus the root cause determination.

The choice of the analytical tools being employed can also be defined by the sample classification. If one observes organic chemistry, the testing methodologies should be tailored toward organic characterization. Similarly, non-organic constituents have a different set of tools for their characterization.

Below are definitions of the different bins included in non-organic contaminants:

Formulation Derived

Many additive components may contribute to deposits either after they have reacted or due to dropping out of solution in an unreacted state. Non-organic formulation-derived deposits typically come from inorganic additive chemistries that have depleted, such as ZDDP.

Inorganics

Inorganic deposits can be defined as those that are free of carbon and are the bin where one includes hard contaminants in the lube, such as dirt, debris and wear metals. It is also common to find inorganic deposits derived from degraded additive components. Depleted ZDDP, for example, can produce inorganic deposits such as phosphates and sulfates.

Soot

High-temperature events may cause the formation of soot particles. Soot deposits consist of black body carbonaceous particles less than 1-micron in size and are typically generated from a dieseling event such as in a diesel engine or in micro- dieseling (the implosion of air bubbles).

Coke

High-temperature events may also produce coke. These are black body carbonaceous materials larger than 1-micron in size. These deposits are typically generated from severe overheating (above 572°F or 300°C) of an organic material until it has released all of its hydrogen and oxygen.

Coal

Coal deposits are also black body carbonaceous materials larger than 1-microns in size, however, they exhibit solubility in some polar organic solvents such as Tetrahydrofuran or Pyridine.

Root Cause Determination

To perform a proper root cause analysis, one should start with a collection of the deposits, the in-service fluid and any known materials that come in contact with the system. Often, but not all the time, the deposit components can also be identified within the in-service fluid. In addition, the overall condition of the fluid tells volumes about the formation direction. For example, if the antioxidant level is in good condition, one should look at something other than simple oxidation as the cause.

After gathering as much data as possible about the fluid condition, we characterize the deposit. A proper analysis of the deposit is often the root cause determination itself. Defining the deposit source classification will help establish the actions to take when fixing the cause. Many times, the deposit contains more than one deposit classification. That information can assist in this root cause determination.

A deposit from a centrifugal compressor used at a petrochemical site in Scandinavia was recently analyzed. The following deposit types were identified in our analysis: inorganic, soot, coke, thermoplastic, thermal degradation, oxidation and lubricant. The deposit types are highlighted in the classification chart in Fig. 6.

Characterizing the different types of chemistries found in the deposit allowed us to understand the various fluid degradation mechanisms in this application. It was also critical in putting together a strategy to address these root causes.

With a root cause and deposit classification assigned, one can direct efforts toward the elimination and removal of the tar-ball deposits. The chemistry of the deposit defines the mitigation strategy since anyone deposit removal technology is not capable of removing all deposits. In addition, it is often beneficial to stop the deposit formation before attempting to remove it.

Conclusion

Lubricant deposits can have a significant impact on a plant’s reliability program. Deposits may have a similar look, leading some to believe that they are all the same in chemistry, originating from the same source. In fact, lube deposits come in a wide range of chemical flavours. Understanding their constituents allows one to better understand why the deposits formed and what remediation strategy to pursue.

This article presented nomenclature and definitions to categorize lube deposits by chemistry and source. In some applications, we find deposits that have components with a wide range of different categories. In other cases, the deposit neatly fits into one categorization bin. In all cases, however, properly characterizing the deposit provides information to allow one to determine the root cause of the problem: the first step to a deposit-free system.